1. Field of the Invention
The present invention relates to a wavelength conversion laser device and, more particularly, to a wavelength conversion laser device which has a nonlinear crystal for generating a second harmonic wave and a structure for rotating the nonlinear crystal for phase matching with a fundamental wave.
2. Description of the Related Art
Recently, there has been an increasing demand for semiconductor lasers in the fields of various displays and light record devices. In particular, as the application range of the semiconductor laser has been expanded to realize full colors in the display field, there has been an increasing demand for lasers having low power-consumption characteristics and capable of high output in a visible ray region.
To obtain red light, AlGaInP or AlGaAs-based semiconductor lasers are relatively easily produced and used. However, in order to obtain green or blue light, it is difficult to grow a semiconductor material due to the unique lattice constant or thermal expansion coefficient of the group III nitride semiconductor. Also, there are problems related to crystal defects such as dislocation, which degrades the reliability and shortens the lifetime of the lasers.
To remedy such problems, a method of converting a wavelength using non-linear characteristics has been used. Diode-pumped Solid-State (DPSS) lasers have gained attention as a method of using the non-linear characteristics. For example, light of a pump laser diode in a band of 808 nm is made to be incident into a crystal like Nd:YAG to obtain a wavelength in the vicinity of 1060 nm, and the frequency is increased by two folds using a nonlinear optical crystal to obtain green light in the vicinity of 530 nm.
In the DPSS laser device, the nonlinear optical crystal such as a crystal for second harmonic generation exhibits refractive index changes due to temperature according to the crystal direction, and thus the incident angle for phase matching, i.e., for optimal wavelength conversion efficiency varies according to the temperature. Therefore, there is required a method for maintaining regulated wavelength conversion efficiency of the non-linear optical crystal within the temperature range of the laser device.
Conventionally, there has been a method of adopting Thermo-electric Cooler (TEC) using a Peltier device and a heat radiating structure, which however increases power consumption or the size of the system. In order to overcome such drawbacks, U.S. Pat. No. 6,614,584 to Govorkov et al. suggests monitoring the light output and displacing the non-linear optical crystal in response to the monitoring result to obtain an incident angle of optimal phase matching condition.
FIG. 1 is a schematic view illustrating a wavelength conversion laser device suggested in the above document.
The wavelength conversion laser device 10 shown in FIG. 1 includes a laser light source 1 and a non-linear optical crystal 5 for converting a wavelength light of the laser light source 1 into a desired wavelength light.
In the wavelength conversion laser device 10, a part of the wavelength light outputted from the nonlinear crystal 5 proceeds to a second beam splitter 2b through a first beam splitter 2a. The second beam splitter 2b separates the light into a vertical component and a horizontal component, and the separated wavelength components are transmitted to first and second location detectors 7a and 7b through a spectral filter 4, respectively. The first and second location detectors 7a and 7b detect the degree of phase mismatching of the light received. According to the detection result, a controller 8 generates displacement γ in the non-linear optical crystal through a rotator 6 so as to obtain an incident angle for optimal output condition.
As described above, the wavelength conversion laser device 10 shown in FIG. 1 monitors the output of light converted by the non-linear optical crystal 6 to provide the degree of phase mismatching due to the current condition (e.g. temperature) as feedback, and according to the result, generates displacement in the non-linear optical crystal 5, thereby maintaining maximum light conversion efficiency.
However, the wavelength conversion laser device 10 has a problem in that the final output position is altered according to the displacement γ of the non-linear optical crystal 5. More specifically, as shown in FIG. 1, when the non-linear optical crystal 5 is displaced (indicated with dotted line), the exit position of light changes from OUT1 to OUT2, causing displacement Δα in the exit position.
Due to such displacement in the exit position, the device has lower precision, and it is difficult to arrange an optical system at the output side. This can cause a serious problem in an ultra-miniaturized product such as a portable projector, which is in the spotlight recently as an application of the laser device.